Expansion POD Protocols

The Expansion POD provides the means
by which BitScope hardware may be extended.

Connected POD devices, including another BitScope, are
controlled using one of two protocols; protocol
pass-through and slow byte exchange.

Protocol Pass-Through

Pass-through makes BitScope "transparent" allowing the host to
communicate with the POD in much the same way as it communicates
with BitScope itself. To establish pass-through, a command byte
to be sent to the POD is first written to the POD transmit
register R18.

The | command is then executed which sends the contents
of the POD transmit register to the POD via IO-O
(serially). BitScope then connects IO-I to the serial port
allowing the POD to send data directly back to the host. This
pass-through remains in place until aborted by a new
command.

Using pass-through, there is a small protocol overhead sending
commands to the POD (ie, programming R18), but there is
none in the other (more important) direction.

Slow Byte Exchange

A second method of POD communication exists for POD devices
that may not be able to handle high speed serial data (or which
may not even know what serial data is !).

In this case, both POD transmit and receive registers
R18/R19 are used.

The command to be sent is put in R18 as above. Upon
execution the x command a UART in BitScope transmits the
byte via IO-O. The UART then monitors IO-I for a reply byte
which when received is placed in R19. The reply byte may
then be retrieved by the host.

If the POD device only understands a single pulse, a command
byte of FF, FE, FC, F8, F0, E0, C0, 80, or 00 can be used.

If no reply is returned by the POD, the next command from the
host aborts the UART and proceeds with the new command.